This invention relates to the removal of impurities found in ethylene glycol recovered from the manufacture of polyethylene terephthalate (PET).
PET is a linear polyester which is generally manufactured in two stages by (1) esterification of terephthalic acid (TPA) with an excess of ethylene glycol (EG) or by the ester exchange reaction of dimethyl terephthalate (DMT) and an excess of EG to form dihydroxyethyl terephthalate (DHET), and (2) the polycondensation of DHET in the presence of a metal oxide catalyst. The metal oxide catalyst is typically an oxide of antimony.
The first stage ester reaction requires an excess of EG. The excess EG is removed during the course of the polycondensation reaction along with other products such as low molecular weight terephthalate oligomers, diethylene glycol (DEG), metal oxide catalysts, typically an oxide of antimony, and trace amounts of other compounds. The EG containing impurities is hereinafter referred to as "spent glycol." The presence of impurities in the spent glycol prevents recycling of the spent glycol into the first stage esterification since product quality would be detrimentally affected thereby. In particular, when product having little or no color is required, spent glycol is unsuitable for recycling.
The prior art on recycling spent glycol relied primarily on flash distillation of the spent glycol as typified by U.S. Pat. Nos. 3,408,268, 3,367,847 and 2,788,373. There are numerous variations to the basic distillation process. For example, U.S. Pat. No. 3,878,085 teaches flash distillation of spent glycol in the presence of an alkali metal hydroxide, while U.S. Pat. No. 3,491,161 teaches the addition of ammonium hydroxide prior to distillation. Some attempts have been made to remove antimony by precipitation prior to distillation of the spent glycol. Typical of these processes are U.S. Pat. Nos. 4,118,582 and 4,013,519.
In practice, spent glycol is purified by distillation in which a pure EG overhead product is recovered. The refined EG is typically sold into the antifreeze market. The still bottoms resulting from the distillation of the spent glycol is a mixture of antimony oxide catalyst, terephthalate oligomers, EG and DEG and various trace impurities such as trace cations, trace anions and color forming impurities.
Significantly, large quantities of still bottoms are generated in the United States each year which presents serious environmental as well as economic problems for PET producers. Furthermore, disposal of the still bottoms as a waste product represents the loss of substantial quantities of EG, DEG, antimony catalysts, and terephthalate oligomers, all of which have commercial value.
There is therefore, a need for a process which removes contaminants from spent glycol and which allows for the recovery and recycle of the still bottoms. Especially desirable is an improved process in which all materials are recycled to either the PET manufacturing process or to other chemical manufacturing processes.